Influence of extreme conditions on the formation and structures of caesium uranium(vi) arsenates

Abstract

Four new uranyl arsenates, Cs2[(UO2)(As2O7)] (1), α-Cs[(UO2)(HAs2O7)] (2), β-Cs[(UO2)(HAs2O7)] (3), Cs[(UO2)(HAs2O7)]·0.17H2O (4), were synthesized by high-temperature/high pressure (HT/HP) reactions at 900 °C and 3 GPa. These phases were subsequently characterised structurally as well as chemically. We demonstrated that compound 1 can also be obtained at ambient pressure. Compounds 1, 2, and 4 are based on two-dimensional (2D) anionic layers with two different topological types. The layers possess a similar composition, [(UO2)(As2O7)](2-) in 1 and [(UO2)(HAs2O7)](-) in 2 and 4. However, the presence of hydrogen in 2 and 4 leads to a change in coordination modes of the pyroarsenate groups. There are additional 0.17 H2O molecules per formula unit in 4, which cause slight distortions of the layers in 4. All these layers can be simplified to a common net, which is typical of autunite-like layered compounds. Compound 3 is a polymorph of compound 2, but the structural arrangements in these two are significantly different. The structure of 3 is based upon a three-dimensional (3D) framework, in which UO7 is coordinated by arsenate groups in order to form uranyl anion sheets, and UO6 is located within the interlayers. Bond valance analysis proved the presence of OH(-) groups in compounds 2, 3, and 4, respectively, and water molecules in 4. The Raman analyses enabled the study of the local environments of the arsenate and the uranyl groups within the investigated phases, respectively. It turned out that the applied HT/HP synthesis method strongly affects the crystal chemistry as well as the observed structural features of all obtained compounds.

Structural and Spectroscopic Investigation of Novel 2D and 3D Uranium Oxo-Silicates/Germanates and Some Statistical Aspects of Uranyl Coordination in Oxo-Salts

“…A large number of peaks have been found in the range of 200–380 cm –1 , which can be assigned to the ν 2 (UO 2 ) 2+ modes. The Raman spectra of 2D layer structures for uranyl oxo-anions such as arsenates and borates also exhibit a large number of bands in the low frequency region. The ν 3 (GeO 4 ) 4– and ν 4 (O–Ge–O) bands appear at 680 and 429 cm –1 , , respectively.…”

Advances and perspectives of actinide chemistry fromex situhigh pressure and high temperature chemical studies

“…As previously described for thorium arsenates and arsenites, the uranium counterparts have been studied extensively using ambient pressure methods owing to the rich chemistry and diverse chemical behaviour that can be extracted from combing the chemistry of arsenic and uranium. Under high temperature ambient pressure and HP/HT conditions Yu et al 71 O of which the latter three are the first examples of uranyl arsenates to be obtained under HP/HT conditions. However, there are yet to be any studies of uranyl arensites resulting from HP/HT conditions.…”

Structural Variations in Complex Sodium Thorium Arsenates

“…The structural chemistry of actinides is very rich due to the complex electronic structure of these elements and consequent variety of oxidation states and diversity of the actinide coordination geometries. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] Thorium, whose redox chemistry, unlike most other actinides, is dominated by the +4 oxidation state, shows highly variable coordination numbers (CN) in inorganic materials ranging from 6 to 15. [17][18][19][20][21][22] Coordination chemistry of Th(IV) is similar to those of highly radiotoxic and long-live transuranic element in the oxidation state +4 (e.g.…”